Retirement Benefits - Gross Accumulated Other Comprehensive Loss and Regulatory Assets (Liabilities) not Recognized as Components of Periodic Benefit Cost (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)		$ 36	$ 49
Actuarial loss		7,310	7,407
Total	[1]	7,346	7,456
Benefits included in accumulated other comprehensive income		4,246	4,310
Benefits included in regulatory assets		3,100	3,146
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit)		(812)	(963)
Actuarial loss		711	985
Total	[1]	(101)	$ 22
Benefits included in accumulated other comprehensive income		(63)
Benefits included in regulatory assets		$ (38)

[1] )Of the $7,346 million related to pension benefits, $4,246 million and $3,100 million are included in AOCI and regulatory assets, respectively, at December 31, 2015. Of the $(101) million related to other postretirement benefits, $(63) million and $(38) million are included in AOCI and regulatory assets (liabilities), respectively, at December 31, 2015. Of the $7,456 million related to pension benefits, $4,310 million and $3,146 million are included in AOCI and regulatory assets, respectively, at December 31, 2014. The $22 million related to other postretirement benefits is included in regulatory assets (liabilities) at December 31, 2014.